Surface compaction versus stretching in Pickering emulsions stabilised by microgels

Amphiphilic Nanogels as Versatile Stabilizers for Pickering Emulsions

Pickering emulsions (PEs) are stabilized by particles at the water/oil interface and exhibit superior long-term stability compared to emulsions with molecular surfactants. Among colloidal stabilizers, nano/microgels facilitate emulsification and can introduce stimuli responsiveness. While increasing their hydrophobicity is connected to phase inversion from oil-in-water (O/W) to water-in-oil (W/O) emulsions, a predictive model to relate this phase inversion to the molecular structure of the nano/microgel network remains missing. Addressing this challenge, we developed a library of amphiphilic nanogels (ANGs) that enable adjusting their hydrophobicity while maintaining similar colloidal structures. This enabled us to systematically investigate the influence of network hydrophobicity on emulsion stabilization. We found that W/O emulsions are preferred with increasing ANG hydrophobicity, oil polarity, and oil/water ratio. For nonpolar oils, increasing emulsification temperature enabled the formation of W/O PEs that are metastable at room temperature. We connected this behavior to interfacial ANG adsorption kinetics and quantified ANG deformation and swelling in both phases via atomic force microscopy. Importantly, we developed a quantitative method to predict phase inversion by the difference in Flory-Huggins parameters between ANGs with water and oil (χwater - χoil). Overall, this study provides crucial structure-property relations to assist the design of nano/microgels for advanced PEs.

Soft gliadin nanoparticles at air/water interfaces: The transition from a particle-laden layer to a thick protein film

HYPOTHESIS

Protein-based soft particles possess a unique interfacial deformation behavior, which is difficult to capture and characterize. This complicates the analysis of their interfacial properties. Here, we aim to establish how the particle deformation affects their interfacial structural and mechanical properties.

EXPERIMENTS

Gliadin nanoparticles (GNPs) were selected as a model particle. We studied their adsorption behavior, the time-evolution of their morphology, and rheological behavior at the air/water interface by combining dilatational rheology and microstructure imaging. The rheology results were analyzed using Lissajous plots and quantified using the recently developed general stress decomposition (GSD) method.

FINDING

Three distinct stages were revealed in the adsorption and rearrangement process. First, spherical GNPs (∼105 nm) adsorbed to the interface. Then, these gradually deformed along the interface direction to a flattened shape, and formed a firm viscoelastic 2D solid film. Finally, further stretching and merging of GNPs at the interface resulted in rearrangement of their internal structure to form a thick film with lower stiffness than the initial film. These results demonstrate that the structure of GNPs confined at the interface is controlled by their deformability, and the latter can be used to tune the properties of prolamin particle-based multiphase systems.

Effective pickering emulsifiers based on submicron carboxymethyl cellulose/chitosan polymer particles

In this study, cross-linked carboxymethyl cellulose/chitosan submicron particles were employed to facilitate the stabilization of Pickering emulsion. The polymer particles were prepared using the polyelectrolyte self-assembly method in conjunction with isocyanide based multicomponent reactions and the characteristics were obtained using: nuclear magnetic resonance, Fourier-transform infrared spectroscopy and dynamic light scattering. Atomic force microscopy revealed the heterogeneous structure of the resulting submicron particles with domains of 20-30 nm in size. The average diameter was found to be in the range of 229-378 nm and they were found to be suitable for the fabrication of oil/water Pickering emulsion when proceeded via the homogenization method followed by sonication. The results obtained revealed that carboxymethyl cellulose/chitosan particles significantly stabilized the droplets at the oil/water interface. Even at low particle concentrations of 0.3 g/L (which is close to that of low molecular weight surfactants) stable Pickering emulsions have been obtained. Additionally, the resulting emulsions showed a high level of stability with regard to changes in pH, temperature and ionic strength. The natural alkaloid piperine was used as a model compound to load the resulting particles, which possessed encapsulation efficiency of 90.6±0.4%. Furthermore, the in vitro release profile of piperine from the Pickering emulsion revealed a much-controlled release in both acidic and neutral media as compared to the unformulated piperine. Additional findings in this work revealed important information on the application of carboxymethyl cellulose/chitosan submicron particles as Pickering stabilizers for creation of new delivery systems.

Interfacial Behavior and Long-Term Stability of the Emulsions Stabilized by Sugar Beet Pectin-Ca2+ Complexes with Different Cross-Linking Degrees

Recent studies showed that sugar beet pectin exhibited more excellent emulsifying properties than traditional citrus peel pectin and apple pectin ascribed to the higher content of neutral sugar, protein, ferulic acid, and acetyl groups. It is precisely because of the extremely complex molecular structure of pectin that the emulsifying properties of the pectin-Ca2+ complex are still unclear. In this study, SBP-Ca2+ complexes with different cross-linking degrees were prepared. Subsequently, their interfacial adsorption kinetics, the resistance of interfacial films to external perturbances, and the long-term stability of the emulsions formed by these SBP-Ca2+ complexes were measured. The results indicated that the highly cross-linked SBP-Ca2+ complex exhibited slower interfacial adsorption kinetics than SBP alone. Moreover, compared with SBP alone, the oil-water interfacial film loaded by the highly cross-linked SBP-Ca2+ complex exhibited a lower elasticity and a poorer resistance to external perturbances. This resulted in a larger droplet size, a lower ζ-potential value, a larger continuous viscosity, and a worse long-term stability of the emulsion formed by the highly cross-linked SBP-Ca2+ complex. This study has very important guiding significance for deeply understanding the emulsification mechanism of the pectin-Ca2+ complex.

Interactions between interfaces dictate stimuli-responsive emulsion behaviour

Stimuli-responsive emulsions offer a dual advantage, combining long-term storage with controlled release triggered by external cues such as pH or temperature changes. This study establishes that thermo-responsive emulsion behaviour is primarily determined by interactions between, rather than within, interfaces. Consequently, the stability of these emulsions is intricately tied to the nature of the stabilizing microgel particles - whether they are more polymeric or colloidal, and the morphology they assume at the liquid interface. The colloidal properties of the microgels provide the foundation for the long-term stability of Pickering emulsions. However, limited deformability can lead to non-responsive emulsions. Conversely, the polymeric properties of the microgels enable them to spread and flatten at the liquid interface, enabling stimuli-responsive behaviour. Furthermore, microgels shared between two emulsion droplets in flocculated emulsions facilitate stimuli-responsiveness, regardless of their internal architecture. This underscores the pivotal role of microgel morphology and the forces they exert on liquid interfaces in the control and design of stimuli-responsive emulsions and interfaces.

Harnessing the polymer-particle duality of ultra-soft nanogels to stabilise smart emulsions

Micro- and nanogels are widely used to stabilise emulsions and simultaneously implement their responsiveness to the external stimuli. One of the factors that improves the emulsion stability is the nanogel softness. Here, we study how the softest nanogels that can be synthesised with precipitation polymerisation of N-isopropylacrylamide (NIPAM), the ultra-low crosslinked (ULC) nanogels, stabilise oil-in-water emulsions. We show that ULC nanogels can efficiently stabilise emulsions already at low mass concentrations. These emulsions are resistant to droplet flocculation, stable against coalescence, and can be easily broken upon an increase in temperature. The resistance to flocculation of the ULC-stabilised emulsion droplets is similar to the one of emulsions stabilised by linear pNIPAM. In contrast, the stability against coalescence and the temperature-responsiveness closely resemble those of emulsions stabilised by regularly crosslinked pNIPAM nanogels. The reason for this combination of properties is that ULC nanogels can be thought of as colloids in between flexible macromolecules and particles. As a polymer, ULC nanogels can efficiently stretch at the interface and cover it uniformly. As a regularly crosslinked nanogel particle, ULC nanogels protect emulsion droplets against coalescence by providing a steric barrier and rapidly respond to changes in external stimuli thus breaking the emulsion. This polymer-particle duality of ULC nanogels can be exploited to improve the properties of emulsions for various applications, for example in heterogeneous catalysis or in food science.

Recent advances in cellulose microgels: Preparations and functionalized applications

Microgels are soft, deformable, permeable, and stimuli-responsive microscopic polymeric particles that are now emerging as prospective multifunctional soft materials for delivery systems, interface stabilization, cell cultures and tissue engineering. Cellulose microgels are emerging biopolymeric microgels with unique characteristics such as abound hydroxyl structure, admirable designability, multiscale pore network and excellent biocompatibility. This review summarizes the fabrication strategies for microgel, then highlights the fabrication routes for cellulose microgels, and finally elaborates cellulose microgels' bright application prospects with unique characteristics in the fields of controlled release, interface stabilization, coating, purification, nutrition/drug delivery, and bio-fabrication. The challenges to be addressed for further applications and considerable scope for development in future of cellulose microgels are also discussed.

Interfacial rheology of model water-air microgels laden interfaces: Effect of cross-linking

HYPOTHESIS

The mechanical properties of model air/water interfaces covered by poly(N-isopropylacrylamide) microgels depend on the microgels deformability or in other words on the amount of cross-linker added during synthesis.

EXPERIMENTS

The study is carried out by measuring the apparent dilational, the compression and the shear moduli using three complementary methods: (1) the pendant drop method with perturbative areas, (2) the Langmuir trough compression, and (3) shear rheology using a double wall ring cell mounted onto a Langmuir through.

FINDINGS

In the range of surface coverages studied, the interfaces exhibit a solid-like behavior and elasticity goes through a maximum as a function of the surface pressure. This is observable whatever the investigation method. This maximum elasticity depends on the microgel deformability: the softer the microgels the higher the value of the moduli. The mechanical behavior of model interfaces is discussed, taking into account the core-shell structure of the particles and their packing at the interface.

Structure formation of PNIPAM microgels in foams and foam films

Responsive aqueous foams are very interesting from a fundamental point of view and for various applications like foam flooding or foam flotation. In this study thermoresponsive microgels (MGs) made from poly(N-isopropyl-acrylamide) (PNIPAM) with varying cross-linker content, are used as foam stabilisers. The foams obtained are thermoresponsive and can be destabilised by increasing the temperature. The structuring of MGs inside the foam films is investigated with small-angle neutron scattering and in a thin film pressure balance. The foam films are inhomogeneous and form a network-like structure, in which thin and MG depleted zones with a thickness of ca. 30 nm are interspersed in a continuous network of thick MG containing areas with a thickness of several 100 nm. The thickness of this continuous network is related to the elastic modulus of the individual MGs, which was determined by atomic force microscopy indentation experiments. Both, the elastic moduli and foam film thicknesses, indicate a correlation to the network elasticity of the MGs predicted by the affine network model.

How Softness Matters in Soft Nanogels and Nanogel Assemblies

Softness plays a key role in determining the macroscopic properties of colloidal systems, from synthetic nanogels to biological macromolecules, from viruses to star polymers. However, we are missing a way to quantify what the term "softness" means in nanoscience. Having quantitative parameters is fundamental to compare different systems and understand what the consequences of softness on the macroscopic properties are. Here, we propose different quantities that can be measured using scattering methods and microscopy experiments. On the basis of these quantities, we review the recent literature on micro- and nanogels, i.e. cross-linked polymer networks swollen in water, a widely used model system for soft colloids. Applying our criteria, we address the question what makes a nanomaterial soft? We discuss and introduce general criteria to quantify the different definitions of softness for an individual compressible colloid. This is done in terms of the energetic cost associated with the deformation and the capability of the colloid to isotropically deswell. Then, concentrated solutions of soft colloids are considered. New definitions of softness and new parameters, which depend on the particle-to-particle interactions, are introduced in terms of faceting and interpenetration. The influence of the different synthetic routes on the softness of nanogels is discussed. Concentrated solutions of nanogels are considered and we review the recent results in the literature concerning the phase behavior and flow properties of nanogels both in three and two dimensions, in the light of the different parameters we defined. The aim of this review is to look at the results on micro- and nanogels in a more quantitative way that allow us to explain the reported properties in terms of differences in colloidal softness. Furthermore, this review can give researchers dealing with soft colloids quantitative methods to define unambiguously which softness matters in their compound.

Multifunctional Silica-Modified Hybrid Microgels Templated from Inverse Pickering Emulsions

Microgels are regarded as soft colloids with environmental responsiveness. However, the majority of reported microgels are inherently hydrophilic, resulting in aqueous dispersions, and only used in water-based applications. Herein, we reported an efficient method for hybridization of poly(N-isopropylacrylamide) microgel by coating hydrophobic silica nanoparticles on their surface. The resultant hybrid microgel had switchable surface wettability and could be dispersed in both aqueous and oil phases. Meanwhile, the coated hydrophobic silica nanoparticles solved the difficulty in redispersing microgels caused by particle aggregation and film formation during the drying process, providing a significant advantage in dried storage. Furthermore, the introduction of hydrophobic silica nanoparticles endowed the hybrid microgel with a variety of applications, including cargo encapsulation, active release induced by emulsion reversion, and trace water absorption.

Two-dimensional colloidal crystal of soft microgel spheres: Development, preparation and applications

Two-dimensional (2D) colloidal crystals are ordered monolayer arrays of colloidal sphere particles assembled on the substrates or at phase interfaces. Owing to their unique periodic structure and fascinating properties, 2D colloidal crystals have aroused considerable interest because of their potential applications. Among them, 2D colloidal crystals self-assembled from soft microgel spheres stand out particularly. The 2D colloidal crystals of soft microgel spheres combine the advantages of monolayer colloidal crystals and sensitive microgels, which have a good application prospect in biomedical area. In this article, we provide a systematic overview of 2D colloidal crystals of soft microgel spheres related to their development, preparation and applications. First, various preparation methods of 2D colloidal crystal of microgels are introduced, including dip-coating, drop-coating, spin-coating, interface assembly, surface reaction-assisted assembly, and so forth. Second, representative biomedical applications consisting of optical sensor, drug delivery, antibacterial coating, cell culture, and colloidal template are also exemplified to show the high performance of 2D colloidal crystals of soft microgel spheres. In addition, we also present prospects of future developments of 2D microgel colloidal crystals.

Thermo-induced inversion of water-in-water emulsion stability by bis-hydrophilic microgels

HYPOTHESIS

Stabilization of water-in-water (W/W) emulsions resulting from the separation of polymeric phases such as dextran (DEX) and poly(ethyleneoxide) (PEO) is highly challenging, because of the very low interfacial tensions between the two phases and because of the interface thickness extending over several nanometers. In the present work, we present a new type of stabilizers, based on bis-hydrophilic, thermoresponsive microgels, incorporating in the same structure poly(N-isopropylacrylamide) (pNIPAM) chains having an affinity for the PEO phase and dextran moieties. We hypothesize that these particles allow better control of the stability of the W/W emulsions.

EXPERIMENTS

The microgels were synthesized by copolymerizing the NIPAM monomer with a multifunctional methacrylated dextran. They were characterized by dynamic light scattering, zeta potential measurements and nuclear magnetic resonance as a function of temperature. Microgels with different compositions were tested as stabilizers of droplets of the PEO phase dispersed in the DEX phase (P/D) or vice-versa (D/P), at different concentrations and temperatures.

FINDINGS

Only microgels with the highest DEX content revealed excellent stabilizing properties for the emulsions by adsorbing at the droplet surface, thus demonstrating the fundamental role of bis-hydrophilicity. At room temperature, both pNIPAM and DEX chains were swollen by water and stabilized better D/P emulsions. However, above the volume phase transition temperature (VPTT ≈ 32 °C) of pNIPAM the microgels shrunk and stabilized better P/D emulsions. At all temperatures, excess microgels partitioned more to the PEO phase. The change in structure and interparticle interaction induced by heating can be exploited to control the W/W emulsion stability.

Engineering hybrid microgels as particulate emulsifiers for reversible Pickering emulsions

Thermo-responsive microgels are unique stabilizers for stimuli-sensitive Pickering emulsions that can be switched between the state of emulsification and demulsification by changing the temperature. However, directly temperature-triggering the phase inversion of microgel-stabilized emulsions remains a great challenge. Here, a hybrid poly(N-isopropylacrylamide)-based microgel has now been successfully fabricated with tunable wettability from hydrophilicity to hydrophobicity in a controlled manner. Engineered microgels are synthesized from an inverse emulsion stabilized with hydrophobic silica nanoparticles, and the swelling-induced feature can make the resultant microgel behave like either hydrophilic or hydrophobic colloids. Remarkably, the phase inversion of such microgel-stabilized Pickering emulsions can be in situ regulated by temperature change. Moreover, the engineered microgels were capable of stabilizing water-in-oil Pickering emulsions and encapsulation of enzymes for interfacial bio-catalysis, as well as rapid cargo release triggered by phase inversion.

Enhanced stability and controlled gastrointestinal digestion of β-carotene loaded Pickering emulsions with particle-particle complex interfaces

In this study, we used large, rigid, and hydrophilic zein-propylene glycol alginate composite particles (ZPCPs) and small, soft, and hydrophobic whey protein microgel (WPM) particles to synergistically stabilize a Pickering emulsion for delivery of β-carotene. The photothermal stability and storage stability of β-carotene were improved with the combined use of different particles. Microstructural observations showed that ZPCPs were effectively adsorbed at the oil/water interface despite the substantial interparticle gaps. WPM particles could swell and stretch on the interface due to their deformable structure, thereby forming an interfacial layer of flattened particles to cover a large surface area. The interfacial structure and macroscopic properties of Pickering emulsions were modulated by adjusting the mass ratio and addition sequence of different particles. The combination of ZPCPs and WPM delayed the lipolysis during gastrointestinal digestion. Through controlling the composition of the complex interface, the free fatty acid (FFA) release rate of Pickering emulsions in the small intestinal phase was reduced from 15.64% to 9.03%. When ZPCPs were used as the inner layer and WPM as the outer layer and the mass ratio of ZPCPs to WPM was 4 : 1, the Pickering emulsion showed the best stability and β-carotene bioaccessibility. The Pickering emulsion with particle-particle complex interfaces could be applied in foods and pharmaceuticals for the purpose of enhanced stability, delayed lipolysis or sustained nutrient release.

Exploring water in oil emulsions simultaneously stabilized by solid hydrophobic silica nanospheres and hydrophilic soft PNIPAM microgel

A general drawback of microgels is that they do not stabilize water-in-oil (w/o) emulsions of non-polar oils. Simultaneous stabilization with solid hydrophobic nanoparticles and soft hydrophilic microgels overcomes this problem. For a fundamental understanding of this synergistic effect the use of well defined particle systems is crucial. Therefore, the present study investigates the stabilization of water droplets in a highly non-polar oil phase using temperature responsive, soft and hydrophilic PNIPAM microgel particles (MGs) and solid and hydrophobic silica nanospheres (SNs) simultaneously. The SNs are about 20 times smaller than the MGs. In a multiscale approach the resulting emulsions are studied from the nanoscale particle properties over microscale droplet sizes to macroscopic observations. The synergy of the particles allows the stabilization of water-in-oil (w/o) emulsions, which was not possible with MGs alone, and offers a larger internal interface than the stabilization with SNs alone. Furthermore, the incorporation of hydrophilic MGs into a hydrophobic particle layer accelerates the emulsions sedimentation speed. Nevertheless, the droplets are still sufficiently protected against coalescence even in the sediment and can be redispersed by gentle shaking. Based on droplet size measurements and cryo-SEM studies we elaborate a model, which explains the found phenomena.

Advances in biopolymeric active films incorporated with emulsified lipophilic compounds: a review

The attention towards active films has increased due to consumer demand for high-quality foods without chemical additives. Active biopolymer-based films have shown great potential for active films by impacting food safety, acting as the carriers of various natural antioxidant and antimicrobial compounds, and decreasing environmental pollution from petrol-derived packaging materials. However, there is a wide range of challenges concerning the different characteristics of biopolymers and plasticizers, often hygroscopic/hydrophilic, compared to numerous lipophilic bioactive compounds. Therefore, recent studies have focused on applying oil-in-water emulsion-based systems to enhance the lipophilic bioactive compounds' dispersibility into the film matrix, improving their performance. It is worth emphasizing that resulting complex systems give rise to new challenges such as (i) dispersion technology of the bioactive compounds with minimum adverse effects on its bioactivities, (ii) interactions between different components of the active films, giving rise to new physicochemical properties, and (iii) the change of the diffusion properties of bioactive compounds into the active films, resulting in different release properties. These challenges are profound and critically discussed in this review, as well as the encapsulation techniques employed in preparing emulsions loaded with lipophilic bioactive compounds for the active film development. An outlook of future directions in the research, development, and application of these active films are given.

Interface-induced hysteretic volume phase transition of microgels: simulation and experiment

Thermo-responsive microgel particles can exhibit a drastic volume shrinkage upon increasing the solvent temperature. Recently we found that the spreading of poly(N-isopropylacrylamide) (PNiPAm) microgels at a liquid interface under the influence of surface tension hinders the temperature-induced volume phase transition. In addition, we observed a hysteresis behavior upon temperature cycling, i.e. a different evolution in microgel size and shape depending on whether the microgel was initially adsorbed to the interface in expanded or collapsed state. Here, we model the volume phase transition of such microgels at an air/water interface by monomer-resolved Brownian dynamics simulations and compare the observed behavior with experiments. We reproduce the experimentally observed hysteresis in the microgel dimensions upon temperature variation. Our simulations did not observe any hysteresis for microgels dispersed in the bulk liquid, suggesting that it results from the distinct interfacial morphology of the microgel adsorbed at the liquid interface. An initially collapsed microgel brought to the interface and subjected to subsequent swelling and collapsing (resp. cooling and heating) will end up in a larger size than it had in the original collapsed state. Further temperature cycling, however, only shows a much reduced hysteresis, in agreement with our experimental observations. We attribute the hysteretic behavior to a kinetically trapped initial collapsed configuration, which relaxes upon expanding in the swollen state. We find a similar behavior for linear PNiPAm chains adsorbed to an interface. Our combined experimental - simulation investigation provides new insights into the volume phase transition of PNiPAm materials adsorbed to liquid interfaces.

Microgels at interfaces, from mickering emulsions to flat interfaces and back

In this review, we cover the topic of p(NIPAM) based microgels at interfaces, revisiting classical studies in light of the newest ones. In particular, we focus on their use as emulsifiers in the so-called mickering emulsions, i.e. Pickering emulsion stabilized by soft particles. Given the complexity of the experimental characterization and simulation of these soft particles at interfaces, the review is structured in progressive complexity levels, until we reach the highly interesting and promising responsiveness to stimuli of mickering emulsions. We start from the lowest level of complexity, the current understanding of the behavior of single microgels confined at a flat interface. Then, we discuss their collective behavior upon crowding, their responsiveness at interfaces, and their macroscopic properties as microgel films. Once we have the necessary characterization tools, we proceed to discuss the complex and convoluted picture of responsive mickering emulsions. The way is rough, with current controversial and contradicting studies, but it holds promising results as well. We state open questions worth of being tackled by the Soft Matter community, and we conclude that it is worth the trouble of continuing after the master theory of microgel interfacial activity, as it will pave the way to widely adopt responsive mickering emulsions as the worthy Pickering emulsion successors.

High internal phase Pickering emulsions stabilized by dialdehyde amylopectin/chitosan complex nanoparticles

High internal phase Pickering emulsions (HIPPEs) have attracted intensive interest for their great potential in foods, cosmetics, and biomedical applications. However, the relatively poor biodegradability and biocompatibility of inorganic and synthetic particulate emulsifiers greatly limit their practical applications. Here, a kind of biobased nanoparticles, namely dialdehyde amylopectin/chitosan complex nanoparticles (DAPCNPs), were fabricated by Schiff base reaction between dialdehyde amylopectin and chitosan with the assistance of ultrasonication treatment. The resultant DAPCNPs were employed to stabilize O/W HIPPEs with various oils, such as toluene, cyclohexane, styrene and edible rapeseed oil. Moreover, the resultant DAPCNPs-stabilized HIPPEs showed high stability under various environmental stresses (80 °C; 20 mM and 100 mM aqueous NaCl solutions). Furthermore, porous scaffolds were also fabricated by freeze-drying cyclohexane-in-water HIPPEs stabilized by DAPCNPs after the introduction of polyvinyl alcohol (PVA) into the continuous phase. These findings would give inspiration for designing polysaccharides-based nanoparticles to stabilize HIPPEs and improve their practical applications.

Microgels self-assembly at liquid/liquid interface as stabilizers of emulsion: Past, present & future

The most recent developments on Pickering emulsions deal with the design of responsive emulsions able to undergo fast destabilization under the effect of an external stimulus. In this scenario, soft colloidal particles like microgels are considered novel class suitable emulsifiers. Microgels particles self-assemblies are highly deformable at interfaces covering higher surfaces than hard particles and their interfacial behavior strongly depends on external-stimuli. Microgels are very diverse owing to the large variety of them from the point of view of possible combinations of stimuli-responsiveness and different microstructures (crosslinking density and distribution). Herein, we illustrate the use of different types of responsive microgels not only from a structural point of view but also even from physical one. For that, the effect of different microgels parameters such as internal structure and charge density on mechanical properties of the interface will be discussed.

Influence of Charges on the Behavior of Polyelectrolyte Microgels Confined to Oil-Water Interfaces

The role of electrostatics on the interfacial properties of polyelectrolyte microgels has been discussed controversially in the literature. It is not yet clear if, or how, Coulomb interactions affect their behavior under interfacial confinement. In this work, we combine compression isotherms, atomic force microscopy imaging, and computer simulations to further investigate the behavior of pH-responsive microgels at oil-water interfaces. At low compression, charged microgels can be compressed more than uncharged microgels. The in-plane effective area of charged microgels is found to be smaller in comparison to uncharged ones. Thus, the compressibility is governed by in-plane interactions of the microgels with the interface. At high compression, however, charged microgels are less compressible than uncharged microgels. Microgel fractions located in the aqueous phase interact earlier for charged than for uncharged microgels because of their different swelling perpendicular to the interface. Therefore, the compressibility at high compression is controlled by out-of-plane interactions. In addition, the size of the investigated microgels plays a pivotal role. The charge-dependent difference in compressibility at low compression is only observed for small but not for large microgels, while the behavior at high compression does not depend on the size. Our results highlight the complex nature of soft polymer microgels as compared to rigid colloidal particles. We clearly demonstrate that electrostatic interactions affect the interfacial properties of polyelectrolyte microgels.

Scavenging One of the Liquids versus Emulsion Stabilization by Microgels in a Mixture of Two Immiscible Liquids

It is known that microgels can serve as soft, permeable and stimuli-responsive alternative of solid colloidal particles to stabilize oil-water emulsions. The driving force for the adsorption of the microgels on interface of two immiscible liquids is a shielding of unfavorable oil-water contacts by adsorbed subchains, that is, the decrease of the surface tension between the liquids. Such phenomenon usually proceeds if volume fractions of the two liquids are comparable with each other and the microgel concentration is not high enough. The natural question arises: what is going on with the system in the opposite case of strongly asymmetric mixture (one of the liquids (oil) has a very small fraction) or high microgel concentration (the overall volume of the microgels exceeds the volume of the minor oil component)? Here we demonstrate that the microgels uptake the oil whose concentration within the microgels can be orders of magnitude higher than outside, leading to the additional microgel swelling (in comparison with the swelling in water). Thus, the microgels can serve as scavengers and concentrators of liquids dissolved in water. At first glance, this effect seems counterintuitive. However, it has a clear physical reason related to the incompatibility of oil and water. Absorption of the oil by microgels reduces unfavorable oil-water contacts by microgel segments: the microgels have a higher concentration of the segments at the periphery, forming a shell. The microgels with uptaken oil are stable toward aggregation at very small oil concentration in the mixture. However, an increase in the oil concentration can lead to aggregation of the microgels into dimers, trimers, and so on. The increasing concentration of oil mediates the attraction between the microgels: the oil in the aggregates appears to be localized in-between the microgels instead of their interior, which is accompanied by the release of the elastic stress of the microgels. A further increase in the oil concentration results in a growth of the size of the oil droplets between the microgels and the number of the microgels at the droplet's periphery, that is, the emulsion is formed.

Poly-N-isopropylacrylamide Nanogels and Microgels at Fluid Interfaces

The confinement of colloidal particles at liquid interfaces offers many opportunities for materials design. Adsorption is driven by a reduction of the total free energy as the contact area between the two liquids is partially replaced by the particle. From an application point of view, particle-stabilized interfaces form emulsions and foams with superior stability. Liquid interfaces also effectively confine colloidal particles in two dimensions and therefore provide ideal model systems to fundamentally study particle interactions, dynamics, and self-assembly. With progress in the synthesis of nanomaterials, more and more complex and functional particles are available for such studies. In this Account, we focus on poly(N-isopropylacrylamide) nanogels and microgels. These are cross-linked polymeric particles that swell and soften by uptaking large amounts of water. The incorporated water can be partially expelled, causing a volume phase transition into a collapsed state when the temperature is increased above approximately 32 °C. Soft microgels adsorbed to liquid interfaces significantly deform under the influence of interfacial tension and assume cross sections exceeding their bulk dimensions. In particular, a pronounced corona forms around the microgel core, consisting of dangling chains at the microgel periphery. These polymer chains expand at the interface and strongly affect the interparticle interactions. The particle deformability therefore leads to a significantly more complex interfacial phase behavior that provides a rich playground to explore structure formation processes. We first discuss the characteristic "fried-egg" or core-corona morphology of individual microgels adsorbed to a liquid interface and comment on the dependence of this interfacial morphology on their physicochemical properties. We introduce different theoretical models to describe their interfacial morphology. In a second part, we introduce how ensembles of microgels interact and self-assemble at liquid interfaces. The core-corona morphology and the possibility to force these elements into overlap upon compression results in a complex phase behavior with a phase transition between microgels with extended and collapsed coronae. We discuss the influence of the internal particle architecture, also including core-shell microgels with rigid cores, on the phase behavior. Finally, we present new routes for the realization of more complex structures, resulting from multiple deposition protocols and from engineering the interaction potential of the individual particles.

Effect of the 3D Swelling of Microgels on Their 2D Phase Behavior at the Liquid-Liquid Interface

We investigate soft, temperature-sensitive microgels at fluid interfaces. Though having an isotropic, spherical shape in bulk solution, the microgels become anisotropic upon adsorption. The structure of microgels at interfaces is described by a core-corona morphology. Here, we investigate how changing temperature across the microgel volume phase transition temperature, which leads to swelling/deswelling of the microgels in the aqueous phase, affects the phase behavior within the monolayer. We combine compression isotherms, atomic force microscopy imaging, multiwavelength ellipsometry, and computer simulations. At low compression, the interaction between adsorbed microgels is dominated by their highly stretched corona and the phase behavior of the microgel monolayers is the same. The polymer segments within the interface lose their temperature-sensitivity because of the strong adsorption to the interface. At high compression, however, the portions of the microgels that are located in the aqueous side of the interface become relevant and prevail in the microgel interactions. These portions are able to collapse and, consequently, the isostructural phase transition is altered. Thus, the temperature-dependent swelling perpendicular to the interface ("3D") affects the compressibility parallel to the interface ("2D"). Our results highlight the distinctly different behavior of soft, stimuli-sensitive microgels as compared to rigid nanoparticles.

Sugar-responsive Pickering emulsions mediated by switching hydrophobicity in microgels

HYPOTHESIS

Pickering emulsions stabilized by soft and responsive microgels can demulsify on demand upon microgel collapse. The concept has been explored with simple model microgels such as poly(N-isopropylacrylamide) (pNIPAM) and their derivatives, but the role of functionalization is largely unexplored.

EXPERIMENTS

Saccharide-responsive phenylboronic-modified microgels are used as Pickering emulsion stabilizers. Emulsion stability and microgel organization at drop surface are studied as a function of saccharide concentration. Better insight into their behavior at interfaces is gained through adsorption kinetics and Langmuir film studies at air-water interface.

FINDINGS

The functionalization of water-swollen microgels by phenylboronic functions imparts some hydrophobicity to the structure, at the origin of additional internal cross-links analogous which rigidify the structure compared to non-functionalized microgels, as proved by their slow adsorption kinetics and poor interfacial compressibility. Upon boronate ester formation with diol groups of the saccharide, the hydrophobic character of the phenylboronic acid decreases, increasing the adsorption kinetics and their interfacial compressibility. Emulsions are stable in the presence of saccharide, given the high deformability of the yet-hydrophilic microgels, and mechanically unstable with less deformable particles in low saccharide concentration. The hydrophobic-hydrophilic switch acts as a trigger to tune the microgel stabilizing properties.

Water-in-Oil Pickering Emulsions Stabilized by Synergistic Particle-Particle Interactions

Here, we report a novel "double Pickering stabilization" of water-in-oil (W/O) emulsions, where complex formation at the interface between Pickering polyphenol particles adsorbing from the oil side and whey protein microgel (WPM) particles coadsorbing from the aqueous side of the interface is investigated. The interfacial complex formation was strongly dependent on the concentration of WPM particles. At low WPM concentrations, both polyphenol crystals and WPM particles are present at the interface and the water droplets were stabilized through their synergistic action, while at higher concentrations, the WPM particles acted as "colloidal glue" between the water droplets and polyphenol crystals, enhancing the water droplet stability for more than 90 days and prevented coalescence. Via this mechanism, the addition of WPM up to 1 wt % gave a significant improvement in the stability of the W/O emulsions, allowing an increase to a 20 wt % water droplet fraction. The evidence suggests that the complex was probably formed due to electrostatic attraction between oppositely charged polyphenol Pickering particles on the oil side of the interface and WPM Pickering particles mainly on the aqueous side of the interface. Interfacial shear viscosity measurements and monolayer (Langmuir trough) experiments at the air-water interface provided further evidence of this strengthening of the film due to the synergistic particle-particle complex formation at the interface.

Microgels at fluid-fluid interfaces for food and drinks

Various aspects of microgel adsorption at fluid-fluid interfaces of relevance to emulsion and foam stabilization have been reviewed. The emphasis is on the wider non-food literature, with a view to highlighting how this understanding can be applied to food-based systems. The various different types of microgel, their methods of formation and their fundamental behavioral traits at interfaces are covered. The latter includes aspects of microgel deformation and packing at interfaces, their deformability, size, swelling and de-swelling and how this affects their surface activity and stabilizing properties. Experimental and theoretical methods for measuring and modelling their behaviour are surveyed, including interactions between microgels themselves at interfaces but also other surface active species. It is concluded that challenges still remain in translating all the possibilities synthetic microgels offer to microgels based on food-grade materials only, but Nature's rich tool box of biopolymers and biosurfactants suggests that this field will still open up important new avenues of food microstructure development and control.

Stimuli-Responsive Behavior of PNiPAm Microgels under Interfacial Confinement

The volume phase transition of microgels is one of the most paradigmatic examples of stimuli-responsiveness, enabling a collapse from a highly swollen microgel state into a densely coiled state by an external stimulus. Although well characterized in bulk, it remains unclear how the phase transition is affected by the presence of a confining interface. Here, we demonstrate that the temperature-induced volume phase transition of poly(N-isopropylacrylamide) microgels, conventionally considered an intrinsic molecular property of the polymer, is in fact largely suppressed when the microgel is adsorbed to an air/liquid interface. We further observe a hysteresis in the core morphology and interfacial pressure between heating and cooling cycles. Our results, supported by molecular dynamics simulations, reveal that the dangling polymer chains of microgel particles, spread at the interface under the influence of surface tension, do not undergo any volume phase transition. The balance in free energy responsible for the volume phase transition is fundamentally altered by interfacial confinement. These results imply that important technological properties of such systems, including the temperature-induced destabilization of emulsions, do not occur via a decrease in the interfacial coverage of the microgels.

Non-Thermoresponsive Decanano-sized Domains in Thermoresponsive Hydrogel Microspheres Revealed by Temperature-Controlled High-Speed Atomic Force Microscopy

Despite the tremendous efforts devoted to the structural analysis of hydrogel microspheres (microgels), many details of their structures remain unclear. Reported in this study is that thermoresponsive poly(N-isopropyl acrylamide) (pNIPAm)-based microgels exhibit not only the widely accepted core-shell structures, but also inhomogeneous decanano-sized non-thermoresponsive spherical domains within their dense cores, which was revealed by temperature-controlled high-speed atomic force microscopy (TC-HS-AFM). Based on a series of experiments, it is concluded that the non-thermoresponsive domains are characteristic for pNIPAm microgels synthesized by precipitation polymerization, and plausible structures for microgels prepared by other polymerization techniques are proposed.

Amphiphilic microgels adsorbed at oil-water interfaces as mixers of two immiscible liquids

Amphiphilic microgels adsorbed at an oil-water interface were studied by means of dissipative particle dynamics (DPD) simulations. The hydrophobic (A) and hydrophilic (B) monomer units in the polymer network are considered to be randomly distributed. Effects of the crosslinking density, interfacial tension between the liquids, their selectivity as solvents towards species A and B, and the degree of incompatibility between the A and B units on the internal microgel structure and distribution of the liquids are considered. The most important predictions are that (i) two immiscible liquids can homogeneously be mixed within the microgels and (ii) the adsorbed microgels contain a high fraction of the liquids (they are swollen at the interface). Simultaneous fulfillment of these two conditions can have a high impact on the design of new and efficient catalytic systems. In particular, such microgels can mix immiscible reactants dissolved in water and oil and trigger chemical reactions in the presence of a catalyst embedded into the microgel.

Controlling the stability of Pickering emulsions by pH-responsive nanoparticles

Electrostatic dissipative particle dynamics simulations were conducted to model the interactions between emulsion droplets stabilized by pH-sensitive polyelectrolyte-grafted nanoparticles. Using a steered molecular dynamics approach, a mechanistic study of forced coalescence was performed to probe the resistance between two particle-covered droplets. The degree of ionization of the grafted polyelectrolytes was adjusted to capture the pH responsiveness. The maximal resistance forces were measured to quantitatively discriminate the efficacy of particles in stabilizing emulsions at different degrees of ionization. Through analyzing droplet dynamics, resistance force variation, and electric field, we discovered that the resistance is attributed to direct electrostatic repulsion, the image charge effect near the water-oil interface, and steric hindrance among extended polymers. When the particle density on the droplet surface is relatively low, the increasing resistance forces at higher degrees of ionization can effectively prevent droplet coalescence. Oppositely, the ionization compromises emulsion stability when the particle surface coverage is high. Substantial desorption of particles from the interface was triggered as the degree of ionization increases. This in turn reduces resistance force and facilitates coalescence. Moreover, the nanoparticles prevent coalescence at high surface coverages by forming dense layers at individual interfaces, while the particle bridges straddling two interfaces were found at low surface coverages, which can also keep the droplets apart.

Super-resolution microscopy on single particles at fluid interfaces reveals their wetting properties and interfacial deformations

Solid particles adsorbed at fluid interfaces are crucial for the mechanical stability of Pickering emulsions. The key parameter which determines the kinetic and thermodynamic properties of these colloids is the particle contact angle, θ. Several methods have recently been developed to measure the contact angle of individual particles adsorbed at liquid-liquid interfaces, as morphological and chemical heterogeneities at the particle surface can significantly affect θ. However, none of these techniques enables the simultaneous visualization of the nanoparticles and the reconstruction of the fluid interface to which they are adsorbed, in situ. To tackle this challenge, we utilize a newly developed super-resolution microscopy method, called iPAINT, which exploits non-covalent and continuous labelling of interfaces with photo-activatable fluorescent probes. Herewith, we resolve with nanometer accuracy both the position of individual nanoparticles at a water-octanol interface and the location of the interface itself. First, we determine single particle contact angles for both hydrophobic and hydrophilic spherical colloids. These experiments reveal a non-negligible dependence of θ on particle size, from which we infer an effective line tension, τ. Next, we image elliptical particles at a water-decane interface, showing that the corresponding interfacial deformations can be clearly captured by iPAINT microscopy.

Correlating the effect of co-monomer content with responsiveness and interfacial activity of soft particles with stability of corresponding smart emulsions

HYPOTHESIS

This study presents the synthesis and characterization of Poly(N-isopropylacrylamide)-co-methacrylic acid (PNIPAM-co-MAA) based multi-responsive soft microgel particles employed as "smart emulsifiers" for controlled stabilization and breakage of the decane-in-water Pickering emulsions. These soft microgel particles can act as reversible stabilizers, i.e. they can either stay at the oil-water interface by supporting emulsion formation or preventing aggregates; while triggering demulsification can be controlled by varying the temperature, pH or ionic strength of the microgel system.

EXPERIMENTS

Dynamic light scattering was applied to observe the variation in hydrodynamic radius of the particles as a function of temperature and pH of the multi-responsive microgel system. Microgel composition was varied in terms of MAA-content and influence of this variation on their thermo-sensitivity and pH responsiveness as well as on the stability of corresponding emulsions was evaluated.

FINDINGS

The microgel particles with highest MAA content showed a significant impact on multi-responsive behaviour. Thermal sensitivity is pH dependent under acidic conditions but this dependence is gradually reduced as the pH increases above 7.5. On the other hand, pH-responsiveness is enhanced with the rise in temperature and stable emulsions were formed under highly alkaline conditions even the temperature was far above the volume phase transition temperature (VPTT). Understanding the correlation of stimuli responsiveness at interface with the emulsion stability would help to fabricate and design novel smart Pickering emulsions with better control over desired properties.